fix incorrect operator lookup for postincrement

[maxbachmann]

Fixes #4535

[da-woods]

In principle this might break some existing code (if they've decided to only wrap one of the operators). I recommend:

1. Add a sentence or two to https://github.com/cython/cython/blob/master/docs/src/userguide/migrating_to_cy30.rst
2. Would it be possible to customize the error message slightly to say something like "wrap c++ operator++(int)" just to help people identify what they need to change?

Point 2 is only if it's an easy change!

[maxbachmann]

In principle this might break some existing code (if they've decided to only wrap one of the operators)

This breaks the current iterator implementation, which only specifies the preincrement operator. I added the postincrement operators in #4528.

Add a sentence or two to https://github.com/cython/cython/blob/master/docs/src/userguide/migrating_to_cy30.rst

I will update the migration guide

Would it be possible to customize the error message slightly to say something like "wrap c++ operator++(int)" just to help people identify what they need to change?

I updated the error message to:

Invalid operand type for '++'. Wrap Foo::operator++(int)
Invalid operand type for '++'. Wrap Foo::operator++()

[da-woods]

Thanks - it's fairly minor breakage and should be easy for users to fix, but hopefully giving them a little hint will make it easier to work out what to do

[maxbachmann]

I just noticed that this does not work for nonmember operators yet.

[maxbachmann]

It would be good to fix #4539 first.

[scoder]

Would it be possible to customize the error message slightly to say something like "wrap c++ operator++(int)" just to help people identify what they need to change?

I updated the error message to:

Invalid operand type for '++'. Wrap Foo::operator++(int)
Invalid operand type for '++'. Wrap Foo::operator++()

It's not clear even to me what "wrap" means here without looking through the code. Did you mean to ask users to add these operators to their external C++ class declarations? Or to correct the existing declarations?

[maxbachmann]

It's not clear even to me what "wrap" means here without looking through the code. Did you mean to ask users to add these operators to their external C++ class declarations? Or to correct the existing declarations?

I think I will update this to be closer to the error message of gcc, which gives the following errors in this case:

error: no 'operator++(int)' declared for postfix '++'
error: no match for 'operator++' (operand type is 'example')

[maxbachmann]

@scoder @da-woods I completely forgot about this PR of mine. I updated the error message and added a paragraph to the Cython 3 migration guide. It would be great to get this in before Cython 3, since this is a least a slightly breaking change.

[da-woods]

It would be good to fix #4539 first.

I think we're prepared to ignore this for now right?

This PR seems a much simpler fix, so is probably worth going in even without the full non member bug fix.

[maxbachmann]

I think we're prepared to ignore this for now right?

Yes I would ignore this issue for now

[maxbachmann]

@da-woods I applied the requested changes

[da-woods]

Thanks @maxbachmann